Sharing ansible variable between plays

This is a follow up to this question.

I have a play that provisions a bunch of EC2 instances. By necessity, the hosts value is localhost (because when it's run there are no hosts), and the play assembles a new host list called ec2hosts, and generates a host ip to hostname map because this is the first and only time that information is made available, and saves it using set_fact. This hosts list is then the subject of a follow-up play. The problem is that I need to use the hostname_map dict created during the provisioning play in the second play, and I can't see how to do this.

Here's the first play:

- hosts: localhost
  connection: local
  gather_facts: False

  tasks:
    - name: Provision a set of instances
      ec2:
        key_name: marcus
        instance_type: t2.micro
        image: "{{ ami_id }}"
        wait: true
        exact_count: "{{ server_count }}"
        count_tag:
          Tutorial: "{{ tutorial_name }}"
        instance_tags:
          Tutorial: "{{ tutorial_name }}"
        groups: ['SSH', 'Web']
      register: ec2

    - name: Add all instance public IPs to host group
      add_host:
        hostname: "{{ item.public_ip }}"
        groups: ec2hosts
      loop: "{{ ec2.instances }}"

    - name: Build an IP to hostname map
      set_fact:
        hostname_map: "{{ hostname_map | combine({item.0.public_ip: (item.1 + '.' + tutorial_domain)}) }}"
      loop: "{{ ec2.instances|zip(hostnames)|list }}"

    - name: Debug hostname_map
      debug:
        msg: "{{ hostname_map }}"

At the end of this, hostname_map contains a map like:

{
    "18.184.109.70": "host1.example.com", 
    "18.196.135.59": "host2.example.com"
}

From reading the ansible docs on variable scope, it says that variables defined in a play are not available outside that play unless it's being applied to the same set of hosts. That's not possible in this case, so I need to use a var with global scope, and from what I've read, set_fact is the appropriate way to do that. So I created an empty variable in /group_vars/all, so that the variable is accessible to all plays:

hostname_map: {}

The next play connects to each newly-created instance (using the host list we created dynamically) and sets its hostname from the inside:

- hosts: ec2hosts
  gather_facts: yes
  tasks:
    - name: Debug hostname_map
      debug:
        msg: "{{ hostname_map }}"
    - name: Set hostnames
      hostname:
        name: "{{ hostname_map[ansible_host] }}"

However, this fails because hostname_map is empty

TASK [Debug hostname_map]
ok: [18.184.109.70] => {
    "msg": {}
}
ok: [18.196.135.59] => {
    "msg": {}
}

so I get this error:

fatal: [18.184.109.70]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: 'dict object' has no attribute u'18.184.109.70'

Curiously I get the same debug output and error even when I don't define the variable globally.

Other articles I've read all suggest that the values should be enumerated explicitly in vars, but I can't do that because the data is dynamic and unknown until runtime. Similarly, I can't provide it using extra_vars on a command line for the same reason.

How can I make this variable available within the second play? I'd like to avoid clunky solutions like writing a local file out and then reading it back in!

I'm also open to suggestions for doing this a completely different way, where "this" is: create an arbitrary number of EC2 instances and assign them hostnames taken from a static list.